Frequency standard



Nov. 22, 1966 P. e. SULZER FREQUENCY S TANDARD Filed Feb. 14, 1962 INNER OVEN CONTQOLLEE OUTEK OVEN CONTROLLER F g INVENTOR PETEZ Gr. SuLzEsz.

ATTORNEYS United States Patent 3,287,658 FREQUENCY STANDARD Peter G. Sulzer, S. Glen Road, Potomac, Md.

Filed Feb. 14, 1962, Ser. No. 173,199

3 Claims. (Cl. 331-69) The present invention relates to frequency standards and more particularly to a highly stable crystal controlled frequency standard.

There is provided in accordance with the present invention a crystal controlled oscillator having a very high degree of frequency stability over relatively extended periods of time and in the presence of relatively large changes in load, and ambient temperature. The principles which have permitted the development of an apparatus having the desired performance characteristics relate to the utilization of two, proportionally controlled ovens in which the oscillator circuit and crystal are located in an inner oven, the inner oven temperature controller and other temperature sensitive elements of the apparatus are located in an outer oven. an AVC transistorized amplifier employed to provide an AVC voltage to the oscillator circuit, this amplifier also employing internal negative feedback to stabilize gain and increase input impedance. Dealing with each of the above principles individually, it has been found that if the oscillator and crystal are placed within a first or inner oven located in a second or outer oven and both of the ovens are proportionally controlled, with the inner oven controller being located in the outer oven, the temperature change in the inner oven has been maintained in an apparatus built in accordance with these principles at roughly of the temperature change measured in the outer oven. This degree of control is attributable to the fact that the inner oven controller is temperature stabilized to a relatively high degree by being located in the outer oven. Further since the inner oven is subject to very small temperature changes, the operating range of the controller is small and therefore may be extremely precise. The proportional controller for the outer oven may be identical with the one for the inner oven and continuing the above example is able to maintain the outer oven temperature variations at less than 1 over a 70 change in ambient temperature. With the inner oven being controlled to maintain temperature variations of roughly %00 of those in the outer oven, the changes in temperature in the inner oven, in which the exceedingly temperature sensitive elements are located, are less than of a degree over a 70 change in ambient temperature. This extreme temperature control is efiected even though the oven controllers are relatively conventional in form and inexpensive.

A second feature of the apparatus relates to the utilization of a transistorized video amplifier to provide the AVG signal for maintaining a constant level of oscillator output signal. By employing an internal negative feedback loop in the video amplifier the gain of the video amplifier is stabilized to a high degree so that the AVG voltage is primarily a function of oscillator output signal uncomplicated by changes due to variations in gain of the video amplifier. Further, the video amplifier provides a high input impedance which is increased and stabilized by the use of negative feedback and therefore the oscillator load is maintained at a fixed high impedance value. The video amplifier is also located within the outer oven and consequently is subject to only very small temperature changes. This fact in conjunction with the negative feedback applied internally of the amplifier has been found to hold the gain of this amplifier and alsoits input impedance at fixed values so that two effects are obtained: The oscillator always looks into a constant high value of These other elements include impedance and the AVG voltage developed by the video amplifier reflects changes in the output voltage of the oscillator per se uncomplicated by changes in the load which it feeds and the gain of the video amplifier developing the AVC voltage. These features insure a constant low power drain on the crystal which is essential to the attainment of high frequency stability.

An oscillator employing the principles of the present invention has been built which is'designed to supply a 50 ohm load. Variations of plus or minus 20% in the value of the resistance of the load produce a change in frequency relative to the frequency for which the apparatus is tuned of i2 10- The same device exhibited a change in frequency relative to the output frequency of :5Xl0 in the presence of temperature changes of C. relative to a basic temperature of 25 C. The same instrument exhibited a change of :5 X 10 in frequency relative to the basic frequency over a period of 24 hours and over a period of one second exhibited-a change of 4 l0 It can be seen from the above that the apparatus of the present invention is intended to provide an extremely stable crystal controlled device and as will become apparent upon subsequent description, the apparatus is relatively simple in construction, quite compact and extremely rugged. Relative to the compactness, the device to be described is contained in a cylinder four inches in diameter and six inches long.

It is therefore an object of the present invention to provide a frequency standard employing a crystal-controlled oscillator in which the crystal and oscillator are located within an inner oven and the inner oven controller is located within an outer oven in which the inner oven is also located.

It is still another object of the present invention to provide a crystal controlled oscillator in which the power drain on the crystal is maintained at a low and constant value as a result of employing a video amplifier having a highly stable gain characteristic and a stable high input impedance.

It is yet another object of the present invention to employ in conjunction with a crystal controlled oscillator an AVC video amplifier employing internal negative feedback to stabilize the gain of the amplifier and increase and stabilize its input impedance.

It is still another object of the present invention to provide a crystal oscillator frequency standard employing proportionally controlled ovens in which the oscillator and its crystal are located in a first oven located internally of a second oven.

. The above and still further objects, features and advantages of the present invention will become apparent upon consideration, of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a schematic block diagram of the basic oscillator of the present invention; and

FIGURE 2 is a partial schematic circuit, partial block diagram of the temperature controller employed in the present invention.

Referring now specifically to FIGURE 1, there is pro vided a first or inner oven 1 located internally of a second or outer oven 2. The oven 1 is a hollow cylinder having removable end walls and a heater winding 3 wound helically about the outer circumference thereof. Similarly, the outer oven 2 is cylindrical and has a helically wound heater 4 disposed about its outer circumference. Located within the inner oven 1 is an oscillator 6 the frequency of oscillation of which is stabilized by a crystal 7. The crystal controlled oscillator 6 may be, for purposes of illustration only a modified Pierce type oscillator, although the specific type of oscillator is notcrit-ical to the invention.

Patented Nov. 22, 1966 The output signal from the oscillator is developed across a pair of leads 8 and applied to an AVC video" amplifier 9 which develops, on an output lead 11, the basic output signal of the apparatus. The video amplifier 9 develops an AVC voltage on a lead 12 which is applied to the oscillator 6 to control the amplitude of the output signal appearing on the lead 8.

The video amplifier 9 is also provided with a negative feedback loop generally designated by the reference numeral 13 which is employed to stabilize the gain of the video amplifier 9. The gain of the amplifier 9 is initially maintained within relatively narrow limits by placing the video amplifier inside of the outer oven 2 so that the amplifier is not subject to extreme temperature variations. This initial stability is further enhanced by employing negative feedback through the loop 13 so that the amplifier 9 is substantially a constant gain device. Further, since a video type amplifier is employed, the input impedance to the amplifier is quite high. If a transistorized amplifier is employed, this providing several advantages such as size, cost and dependability, then the negative feedback increases the input impedance and stabilizes it along with the gain of the amplifier.

The above arrangement provides two distinct and important advantages in the operation of the apparatus. The fixed high input impedance to the video amplifier 9 maintains a constant load on the crystal 7 which is essential to the high stability of the output frequency. Secondly, due to the fact that the gain of the video amplifier is stabilized the AVG voltage appearing on the lead 12 refiects changes intern-ally of the oscillator 6 only and does not reflect to any appreciable extent changes in characteristics of the video amplifier 9.

The inner and outer ovens 1 and 2, respectively, are controlled by identical oven controllers designated respectively by the reference numerals 14 and 16. The inner oven has located therein a thermistor 17 serving as the sensing element for the controller 14. The power output signal of the controller 14 drives a heater 3, directly. The controller 14 is a proportional control device as is the controller 16 so that a very high degree of temperature regulation is possible. The outer oven controller 16 employs a thermistor 18 as its sensing element and the output energy from the controller 16 is employed to 'drive the heater 4.

It has been found that as a result of the arrangement of the inner oven within the outer oven and the location of the inner oven controller within the outer oven that a very high degree of temperature stability may be maintained within the inner oven 1. As previously indicated, the variation which may be maintained within the oven 1 is less than of a degree over ambient temperature variations of 70 C.

This high degree of control results from the fact that the outer oven is controlled to variations of less than one degree and therefore the controller 14 sulfers little from temperature effects. As will become apparent from a description of FIGURE 2, the controller 14 includes a resistance bridge for detecting changes in impedance in the thermistor element. The resistors employed in the bridge are chosen for their temperature stability and thus the input signal to the amplifier section of the controller is, for all practical purposes, a function only of the changes in temperature of the inner oven. Obviously, the temperature stability in the outer oven also stabilizes the gain of the amplifier section of the controller 14 so that the overall effect of this arrangement is to provide extremely high temperature stability in the inner oven in which are located only these elements which are extremely sensitive to temperature variations. Not only does this arrangement provide a temperature stabilized environmnt for the oscillator and crystal but, since the crystal and oscillator are quite small and the inner oven may be small, temperature gradients in the inner oven are minimized. This cannot be true to the same extent in the larger outer oven but the elements located therein are not appreciably effected by these relatively minor effects.

Referring now specifically to FIGURE 2 of the accom panying drawings, the temperature controller is illustrated. The controller comprises a three stage amplifier employing positive feedback. When the bridge is unbalanced due to temperature changes within the oven 1 or oven 2 as the case may be, the feedback produces oscillation of the circuit and the amplitude of the oscillation is a function of the degree of unbalance of the bridge. The three stage amplifier employed in the apparatus comprises a first stage of amplification including NPN transistor 19, a second stage of amplification including the NPN transistor 21 and a third stage of amplification including the NPN transistor 22.

A primary winding 23 of a transformer 24 is connected in the collector circuit of the transistor 22 as its load. Transformer 24 has a center-tapped secondary winding 26 and connected across the secondary winding is a variable resistor 27, a fixed resistor 28 and the thermistor 18 if this is the inner oven controller and thermistor 18 if it is the outer oven controller. The output voltage of the bridge is taken between leads 29 and 31, the former being connected to the center tap of the secondary winding 26 and the latter being connected between resistor 28 and thermistor 18 or 17. The lead 31 is returned to the base electrode of the transistor 19 to provide a positive feedback loop to the input of the three stage amplifier. Operating voltage for the circuit is derived from a bus 32 and a second bus 34 is connected to a suitable source of refer ence potential, such as ground. The output signal of the amplifier is developed at the collector of the transistor 22 and is applied to an output terminal 33. After suitable amplification this signal is employed to drive the heater 3 or 4 as the case may be.

In operation if the bridge comprising the secondary Winding 26 of transformer 24, resistors 27 and 28 and thermistor 18 or 17 is balanced, no output voltage is developed on the output terminal 33. However, if the bridge is unbalanced, a voltage is developed between the leads 29 and 31 and since this is a positive feedback amplifier having a loop gain of one or slightly greater, the circuit goes into oscillation. The amplitude of the oscillation is a function of the degree of the imbalance in the bridge circuit and therefore the output signal is a function of the degree of imbalance. As previously indicated, the AC. output voltage developed'at the terminal 33 is applied via suitable amplifiers to the heater which corrects the change in temperature detected by the thermistor.

The temperature controller is of completely conventional operation and it is to be seen that it requires a threestage amplifier and a bridge circuit with a feedback from the bridgeto the amplifier so as to produce oscillation upon imbalance of the bridge. The .same controller is employed for both ovens and the novelty in the temperature control does not reside primarily in the circuit but in the utilization of two proportional controllers for two related ovens in which the inner oven controller is located in the outer oven.

The AVC video amplifier is also of conventional design. The AVC voltage is derived through a rectifier-filter arrangement coupled to a signal circuit of the amplifier so as to derive the negative voltage required to stabilize the oscillator output signal. The negative feedback voltage for the video amplifier is derived directly from the output voltage of the amplifier and is applied to the emitter electrode of a common emitter connected transistor serving as the input stage of the amplifier.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. A frequency standard comprising a temperature stabilized outer oven, a temperature stabilized inner oven located in said outer oven, a crystal controlled oscillator located in said inner oven, a high input impedance amplifier for amplifying output Waves generated by said oscillator, said amplifier being located in said outer oven and including negative feedback means for stabilizing its gain, means responsive to the output of said amplifier for deriving an automatic volume control signal indicative only of the amplitude of said waves as derived from said amplifier, and means responsive to said automatic volume control signal to stabilize the amplitude of the output waves generated by said oscillator.

2. A frequency standard comprising an outer oven, an inner oven, an oscillator, a piezoelectric crystal connected to said oscillator to stabilize the oscillatory frequency of said oscillator, said oscillator and crystal being located in said inner oven, means including a temperature sensitive element for deriving a first signal indicative of the deviation of the temperature of said inner oven from a desired temperature, means responsive to said first signal for stabilizing the temperature of said inner oven at said desired temperature, said means for deriving, exclusive of said temperature sensitive element, being located in said outer oven, means including a temperature sensitive element for deriving a second signal indicative of the deviation of the temperature of said outer oven from a desired temperature, means responsive to said second signal for stabilizing the temperature of said outer oven at said desired temperature, a transistorized amplifier having a high input impedance and including a negative feedback circuit to stabilize its gain and increase and stabilize its input impedance, said amplifier being located in said outer oven, means conmeeting said amplifier to receive the output waves generated by said oscillator, mean-s for deriving an automatic gain control signal indicative only of the amplitude of said waves as derived from said amplifier and means for applying said automatic gain control signal to said oscillator to stabilize the amplitude of said output waves.

3. The combination according to claim 2 wherein said first mentioned temperature sensitive element is a temperature variable impedance located in said inner oven,

References Cited by the Examiner UNITED STATES PATENTS 1,959,156 5/1934 Downey 331--69 2,169,307 8/1939 Tunick 33169 X 2,775,699 12/1956 Felch 331-70 X 3,007,023 10/ 1961 Johnston et a1 219-210 3,084,294 4/ 1963 Vallese 331109 X 3,109,082 10/1963 Polaniecki 331-69 X ROY LAKE, Primary Examiner.

I. B. MULLINS, Assistant Examiner. 

1. A FREQUENCY STANDARD COMPRISING A TEMPERATURE STABILIZED OUTER OVEN, A TEMPERATURE STABILIZED INNER OVEN LOCATED IN SAID OUTER OVEN, A CRYSTAL CONTROLLED OSCILLATOR LOCATED IN SAID INNER OVEN, A HIGH INPUT IMPEDANCE AMPLIFIER FOR AMPLIFYING OUTPUT WAVES GENERATED BY SAID OSCILLATOR, SAID AMPLIFIER BEING LOCATED IN SAID OUTER OVEN AND INCLUDING NEGATIVE FEEDBACK MEANS FOR STABILIZING ITS GAIN, MEANS RESPONSIVE TO THE OUTPUT OF SAID AMPLIFIER FOR DERIVING AN AUTOMATIC VOLUME CONTROL SIGNAL INDICATIVE ONLY OF THE AMPLITUDE OF SAID WAVES AS DERICED FROM SAID AMPLIFIER, AND MEANS RESPONSIVE TO SAID AUTOMATIC VOLUME CONTROL SIGNAL TO STABILIZE THE AMPLITUDE OF THE OUTPUT WAVES GENERATED BY SAID OSCILLATOR. 